T Tauri stars like the young Sun have far stronger stellar winds than more stable, older stars. Uranus and Neptune are thought to have formed after Jupiter and Saturn did, when the strong solar wind had blown away much of the disc material. As a result, the planets accumulated little hydrogen and helium-not more than 1 Earth mass each. Uranus and Neptune are sometimes referred to as failed cores.[ E. W. Thommes, M. J. Duncan, H. F. Levison (2002). "The Formation of Uranus and Neptune among Jupiter and Saturn". Astronomical Journal 123 (5)： 2862.] The main problem with formation theories for these planets is the timescale of their formation. At the current locations it would have taken a hundred million years for their cores to accrete. This means that Uranus and Neptune probably formed closer to the Sun-near or even between Jupiter and Saturn-and later migrated outward (see Planetary migration below).[ Harold F. Levison, Alessandro Morbidelli, Crista Van Laerhoven et al. (2007). "Origin of the Structure of the Kuiper Belt during a Dynamical Instability in the Orbits of Uranus and Neptune". Icarus 196 (1)： 258.] Motion in the planetesimal era was not all inward toward the Sun; the Stardust sample return from Comet Wild 2 has suggested that materials from the early formation of the Solar System migrated from the warmer inner Solar System to the region of the Kuiper belt.[ Emily Lakdawalla (2006). "Stardust Results in a Nutshell: The Solar Nebula was Like a Blender". The Planetary Society. Retrieved 2007-01-02.]

　　Based on recent computer model studies, the complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth. According to the computer studies, this same process may also occur around other stars that acquire planets.[ Moskowitz, Clara (29 March 2012). "Life's Building Blocks May Have Formed in Dust Around Young Sun". Space.com. Retrieved 30 March 2012.]

　　After between three and ten million years, the young Sun's solar wind would have cleared away all the gas and dust in the protoplanetary disc, blowing it into interstellar space, thus ending the growth of the planets.[ B. G. Elmegreen (1979). "On the disruption of a protoplanetary disc nebula by a T Tauri like solar wind". Astronomy & Astrophysics 80: 77.][ Heng Hao (24 November 2004). "Disc-Protoplanet interactions". Harvard University. Retrieved 2006-11-19.]

　　TOPIC 中世纪商人行会

　　Merchant Guilds in the Middle Ages[ http://www.middle-ages.org.uk/merchant-guilds-in-the-middle-ages.htm]

　　The guilds in the Middle Ages were an important part of Medieval life. A higher social status could be achieved through membership to Merchant guilds. There were two main kinds of Medieval guilds - Merchant Guilds and Craft Guilds. The word "guild" is from the Saxon "gilden" meaning "to pay" and refers to the subscription paid to the Guilds by their members。

　　The system of Feudalism during the Middle Ages allowed the lords and owners of the land to tax the people and their trades. As trade increased in the Middle Ages the taxes became excessive. A single person had no chance of making any objections to the rate and amount of tax that the lord demanded. The idea of Merchant Guilds was born. A Merchant Guild was an association of of traders. The Merchant Guild was able to negotiate with the lord and the trade levy became regulated. The regulations agreed between the Merchant Guild and the lord resulted in a Merchant Guild charter. The Merchant Guild charter allowed the merchants to pay an annual payment, or fixed sum, to the lord who owned the land where the town was based。

　　Rules of the Merchant Guilds during the Middle Ages

　　The members of the Merchant Guilds became powerful. The Merchant Guilds controlled the way in which trade was conducted in the town. The merchant Guilds applied rules to the way in which trade was conducted during the Middle Ages. These rules were included in the charters of the Merchant Guilds and included:

　　A ban on, or fines imposed, on any illicit trading by non Merchant Guild members

　　Fines were imposed on any Merchant Guild members who violated the Merchant Guilds charter

　　Members of the Merchant Guilds were protected and any Merchant Guild member who fell sick was cared for by the guild. Burials of guild members were arranged and the Merchant Guilds undertook to care for any orphans

　　The members of Merchant Guilds also provided protection of their horses, wagons, and goods when moving about the land as travelling during the Middle Ages was dangerous

　　Members of the Merchant Guilds in the Middle AgesThe leading members of the Merchant Guilds became very important members of the Medieval town community of the Middle Ages. Leading Merchant Guild members adopted the role of spokesperson for all of the members. The introduction of the Merchant guilds in a town or city lead to its own hierarchy and involvement in civic duties:

　　The chief spokesman of a Merchant Guild became the mayor of the town, or city

　　The leading delegates of the Merchant Guilds became the Aldermen of the town or city

　　The other members of the Merchant Guilds became the burghers of the town or city

　　The power of the Merchant Guild members increased to such an extent that the livelihood of individual trades or crafts within a Medieval town, or city, were being jeopardised. The Merchant Guilds were imposing regulations on the individual traders or craftsmen to regulate prices and supply. The individual workers of trades or crafts followed the example of the Merchant Guilds who had objected to the lords of land and in turn raised objections to the Merchant Guilds.   The individual crafts and trades established their own guilds. The Craft Guilds were then established in the Medieval town or city of the Middle Ages。

　　The craft guilds

　　In contrast to the land-bound serfs, townspeople of the Middle Ages were free. Some engaged in commerce and formed groups known as merchant guilds. The majority, however, were small merchant-craftsmen, organized in craft guilds as masters (of highest accomplishment and status)， journeymen (at a middle level)， and apprentices (beginners). The medieval master was typically many things at once: a skilled workman himself; a foreman, supervising journeymen and apprentices; an employer; a buyer of raw or semifinished materials; and a seller of finished products. Because medieval craftsmen employed simple hand tools, a workman's own skill determined the quantity and quality of his output. Apprentices and journeymen underwent long periods of learning under the guidance of a more experienced workman. When he could produce a "masterpiece" that met the approval of the guild masters, the craftsman would gain full admission into the guild。

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